Magnesium batteries are theoretically capable of achieving up to five times higher volumetric energy densities compared to Li ion batteries. The enhanced volumetric capacity is enabled by the use of a Mg metal anode that undergoes Mg electrodeposition and stripping processes during battery cycling. Fortunately, Mg electrodeposition largely results in morphologically smooth surfaces that bypass the risk of battery shorts due to dendrite formation. The challenge, therefore, lies in the development of electrolyte solutions from which Mg electrodeposition and stripping readily occurs. Mg metal is highly reactive and thus precludes the use of conventional battery electrolyte components such as carbonates. Currently, most electrolyte solutions from which Mg electrodeposition and stripping occur almost always contain chloride including the traditional Grignard and organohaloaluminate electrolytes in addition to the more recently developed all-inorganic magnesium aluminum chloride complex (MACC) electrolyte.

Here we describe a multimodal approach to explore the activity of chloride in Mg electrolyte solutions. The MACC electrolyte solution is taken as a model electrolyte system upon which a variety of spectroscopic techniques are applied. The benefit of MACC lies in the electrolytic conditioning process that is required to activate the electrolyte providing both an “off” state and “on” state from which relative changes can be elucidated. Raman spectroscopy, ²⁷Al NMR, ³⁵Cl NMR, and pair distribution function analysis reveal the change in the Mg and Al related complexes in the as-prepared (off) versus conditioned (on) electrolyte solution. The Al concentration drops as the electrolyte is conditioned due to the irreversible Al plating occurring during the initial conditioning. The drop in Al concentration is accompanied by an increase in Mg concentration due to oxidation of the Mg counter electrode. The Mg complexes in both the as-prepared and conditioned electrolyte, however, are confirmed to be the elusive Mg dimer complex [Mg₂(μ-Cl)₃·6THF]⁺ commonly implicated as the active Mg complex. The Mg dimer is a common moiety found in the solid state crystal structures of dried Mg electrolytes, however, this is the first time the Mg dimer has been characterized in solution. The fact that the dimer is present in both the inactive and active electrolyte suggests that its mere presence is not the cause for increased activity. Instead, the formation of free chloride in the MACC electrolyte is responsible for enhanced Mg electrodeposition and stripping as observed by surface enhanced Raman spectroscopy.

The ability of chloride to promote Mg electrodeposition and stripping has important implications for the development of Mg battery electrolytes and cathode materials. Recently, the field has been moving away from electrolytes that contain chloride due to the inherent instability of oxide cathodes with chloride-containing electrolytes. The role of chloride in the Mg electrodeposition and stripping processes must be elucidated further to determine if alternative enhancing agents can be designed.